Thermosetting epoxy resin powders are widely used to provide protective coatings, especially for steel pipe. The coatings may be applied by blowing the powder onto pipe which has been preheated to a temperature above the softening point of the powder so that the powder fuses and adheres to the pipe. The pipe may then be carried to an oven to cure the resin, or if the pipe contains sufficient heat, the resin fully cures before the pipe has cooled to room temperature.
The cured coatings should be sufficiently adherent and flexible to withstand the bending and impact to which the pipe or other substrate may be subjected during shipment and installation. Insufficient flexibility becomes especially troublesome in cold weather. In use, the cured coatings should be resistant to the most adverse conditions that may be encountered. For example, cured coatings are subject to disbondment due to the cathodic protection which is commonly applied to buried pipe, a condition aggravated in hot service, especially when the pipe is buried in moist ground.
For commercial utility, it is necessary that the uncured powder be stable at room temperature. Upon striking the heated substrate, the uncured powder must fuse and flow sufficiently to provide pinhole-free coatings, but should cure to a thermoset state quickly at moderately elevated temperature to permit the coatings to be applied at reasonably high rates of production. For some applications the coating should harden at the curing temperature sufficiently to permit handling within ten seconds, although longer times are satisfactory in many applications.
Protective coating powders which generally satisfy the foregoing criteria are known in the prior art. For example, the epoxy resin powders of U.S. Pat. No. 3,578,615 (Moore) provide protective coatings having improved flexibility and resistance to cathodic disbondment. The epoxy resin powder of U.S. Pat. No. 3,876,606 (Kehr) is equally good in such respects while providing extraordinarily good resistance to boiling water, an important property for pipe which is to be buried in moist ground and used to convey hot fluids. Nevertheless, there has been a continuing need for further improvement in flexibility, especially at sub-freezing temperatures.
Although flexible thermoplastic materials such as butadiene/acrylonitrile copolymers have been used in epoxy resin powders to improve the cured flexibility, such modifications have primarily concerned powdered epoxy adhesives, e.g., U.S. Pat. No. 3,707,583 (McKown). It has generally been believed that protective coatings including unreacted thermoplastic materials would be deficient in impact resistance and in resistance to cathodic disbondment. Accordingly efforts to incorporate flexibilizers have usually involved those which form chemical bonds with an ingredient of the epoxy resin composition. For example, U.S. Pat. No. 3,639,345 (Whittemore) pre-reacts polyethylene or polypropylene glycol with the anhydride curing agent. Such procedures can be difficult and expensive.
Recently the company to which this application is assigned sold epoxy resin powder comprising 100 parts of a polyglycidyl ether of a polyhydric phenol having a softening point of 70.degree.-150.degree. C. and 5-10 parts by weight of a copolymer of acrylonitrile, i.e., "nitrile rubber", plus latent hardening agent and accelerator. This powder provided protective coatings which had improved flexibility and were equivalent in other respects to less-flexible coatings of the prior art except that the nitrile rubber inhibited flow-out so that the coatings were rough and, if thin, contained pinholes.